1. Technical Field
The present invention relates to a reflection reducing film which is formed on one face of an optical member such as a lens, a filter and the like to exhibit a reflection reducing effect with respect to light having a predetermined wavelength band, and an optical member and an optical system which have the reflection reducing film.
2. Related Art
Generally, an image pickup apparatus such as a photographic camera or a TV broadcasting camera has a number of optical members such as lenses, prisms, filters and the like disposed on an optical path thereof. When light is incident on surfaces of the respective optical members, a portion of the light becomes reflection light. Herein, when the number of the optical members increases, a total amount of the reflection light increases and thus defects, for example, flare and ghost occur in images of a broadcasting camera. In addition, since reflectances on the surfaces of the optical members have distribution with respect to a wavelength of the incident light and wavelength dependences of various reflectances are shown in accordance with constituent materials for the respective optical member, a chromaticity balance deteriorates, and thus it is necessary to adjust a white balance in an entire image pickup apparatus.
Accordingly, a surface of each optical member may be provided with a reflection reducing film (also referred to as the antireflection film). The reflection reducing film is a multilayer film in which dielectric films having different refractive indices are combined, and a configuration thereof is disclosed in, for example, the following Non-Patent Document 1. In the Non-Patent Document 1 (“Light Thin Film Manual”, The Optronics Co., Ltd, Oct. 9, 1989, P 246-247), an antireflection film having a 5-layer structure is disclosed and an attempt has been made to achieve a low reflectance over a wider band.
In addition to the Non-Patent Document 1, Patent Document 1 (JP-A-2002-267801) discloses an antireflection film having a 9-layer structure.
For example, the antireflection film disclosed in the above Non-Patent Document 1 exhibits relatively excellent low reflectance characteristics when being provided on an optical substrate having a refractive index lower than 1.70 at the d-line. However, it is found that there is a tendency in which flatness in reflectance distribution is lost as described later and a reflectance is thus increased at a specific wavelength when the antireflection film is provided on an optical substrate having a high refractive index higher than 1.75.
Tables 1 and 2 show basic data (constituent materials, refractive indices and optical film thicknesses) of antireflection films (Related examples 1 and 2) having a laminate structure corresponding to the antireflection film disclosed in Non-Patent Document 1. And FIG. 45 shows reflectance distributions of the antireflection films having the basic data of Tables 1 and 2.
TABLE 1CENTER WAVELENGTH λ0 = 500 nmOPTICAL FILMRELATEDCONSTITUENTREFRACTIVETHICKNESSEXAMPLE 1MATERIALINDEX NINDEX N × dOPTICALBK-71.51—SUBSTRATE1st LAYERSUB-M11.670.25 λ 02nd LAYERZrO22.050.50 λ 03rd LAYERAl2O31.620.50 λ 04th LAYERSUB-H42.050.50 λ 05th LAYERMgF21.390.25 λ 0AIR—1.00—
TABLE 2CENTER WAVELENGTH λ0 = 500 nmOPTICAL FILMRELATEDCONSTITUENTREFRACTIVETHICKNESSEXAMPLE 2MATERIALINDEX NINDEX N × dOPTICALS-LAH581.88—SUBSTRATE1st LAYERSUB-M31.780.25 λ 02nd LAYERZrO22.050.50 λ 03rd LAYERAl2O31.620.50 λ 04th LAYERSUB-H42.050.50 λ 05th LAYERMgF21.390.25 λ 0AIR—1.00—
Related example 1 of Table 1 is a constitutional example when a refractive index of an optical substrate on which the antireflection film is formed is relatively low, and Related example 2 of Table 2 is a constitutional example when a refractive index of an optical substrate is relatively high. In Tables 1 and 2, “SUB-M1” represents a substance M1 (Merck & Co., Inc.) including PrAlO3 as a main component, “SUB-M3” represents a substance M3 (Merck & Co., Inc.) including lanthanum aluminate (La2xAl2YO3(X+Y)) as a main component, and “SUB-H4” represents a substance H4 (Merck & Co., Inc.) including LaTiO3 as a main component. In FIG. 45, a curved line 23A indicates the reflectance distribution of Related example 1 and a curved line 23B indicates the reflectance distribution of Related example 2. In Related example 2 in which the refractive index of the optical substrate is 1.8830 as above, the reflectance distribution is not flat and peaks are generated at wavelengths of about 420 nm, 510 nm and 650 nm.
Moreover, recently, demand for a day and night vision camera (day-night-camera) as a broadcasting camera increases. However, the antireflection film described in the above Patent Document 1 has insufficient antireflection performance in a near-infrared region. Thus, an optical system capable of excellently performing photographing in a near-infrared region at a wavelength of about 700 to 900 nm in addition to in a visible light region is strongly required.
Accordingly, a reflection reducing film, which has a sufficiently low reflectance for light at a wavelength of about 400 to 900 nm even when being provided on an optical substrate having a relatively high refractive index, is desired.